1. Field of Invention
The present invention relates to a method and apparatus for producing a semiconductor device. More particularly, the present invention relates to a vertical type apparatus for producing, by using a silicon- or compound-semiconductor, devices such as a memory device, a logic circuit IC, a thin-film transistor IC, and the like, as well as relates to a method for producing a semiconductor device in such an apparatus.
In such an apparatus, a film or a layer consisting of semiconductor material, insulating material, metallic material, super-conducting material or the like is formed on a wafer using a reaction gas under normal or reduced pressure. Such heat treatments as diffusion, improvement of the film properties, and flattening of a film are also carried out under the presence of a protective gas. The present invention provides a method for decreasing the particles in such an apparatus and method as described above.
The "reaction gas" herein means, as usual: gases which react with one another to form the film or layer; gas or gases which decompose to form the film or layer; gas or gases which react with a wafer to form the film or layer. The gas or gases mean not only the atom or molecule as usual but also the species in the state of ions or radicals. The "inert gas" herein means the gas, whose reaction with the wafer is not intended or which does not impede the function of the reaction gas as described above. The inert gas may be Ar or the like or may be nitrogen which does not react with a wafer under a certain kind of the heat treatment.
2. Description of Related Arts
Japanese Unexamined Patent Publications Nos. 53-51187 and 65-20282 disclose apparatuses for producing a semiconductor device. These apparatuses are rather old and incur a considerable temperature fall at one of the ends of a reaction tube, i.e., the exhaust end. The film incidentally formed on the inner wall of the exhaust end has a quality consierably different from the one formed on a wafer as a result of the reaction. As a result, the particles sprinkle in the reaction chamber.
In the late 1970s and early 1980s, an evacuated CVD apparatus usually referred to as Anicon was frequently used. In this apparatus, a vertically movable upper furnace of an electrical resistance heating type and a stationary lower heating furnace are fixed air-tight to one another, but are also separable. An exhaust tube is aligned to the central axis of the upper and lower furnaces. The lower furnace is fixed to a lower portion of the exhaus tube. The upper portion of the exhaust tube protrudes into the inner space of the upper furnace. Small inlet tubes of reaction gas are fixed in the exhaust tube concentrically. A wafer carrier made of quartz is held above the exhaust tube in a central portion of the furnace. The wafer carrier is in the form of a drain board and can carry a number of wafers. The attained dispersion in thickness of the film formed on the wafers is .+-.2% which is excellent in the above-mentioned era. However, when the upper and lower furnaces are separated to take out the wafers, a great temperature difference is generated between the interior and exterior of the apparatus. A considerable hot air blast therefore generates and deposits particles of reaction product particles on the wafers in a great amount.
Contrary to this, a dual-tube type apparatus for producing a semiconductor device is recommended, because the generation of particles is very slight. More specifically, in the dual-tube type apparatus the outer tube has a top closed except for the gas inlet where a gas conduit opens, and an annular channel is formed between the inner and outer tubes. The annular channel is communicated with the tubular space of the inner tube via a space above the top end of the inner tube. According to the so-formed furnace structure, the reaction gas is withdrawn through the annular channel, and almost all of the particles can be confined in this annular channel, as is disclosed in European Patent Publication 0538874 filed by the present applicant.
FIG. 17 is a drawing illustrating a conventional method and apparatus for producing a semiconductor device using a dual-tube type apparatus.
Referring to FIG. 17, a method for heat treating a wafer in an apparatus disclosed in said European publication is illustrated. A dual reaction-tube 1 comprises an outer tube 1a and an inner tube 1b. An inlet conduit of the reaction gas 2 opens at the bottom of the inner tube 1b. The exhaust tube 3 opens at the bottom of the outer tube 1a. The annular channel 4 is formed between the concentrical, outer tube 1a and inner tube 1b and guides the reaction gas into the exhaust tube 3. The heating furnace is denoted by 5. The wafer holder is denoted by 6 and holds a wafer 8. The wafer 8 held at the position shown in FIG. 17 is brought into contact with and caused to react with the reaction gas which is caused to flow upward in the inner tube 1b. The intended reaction thus occurs. After the reaction, the reaction gas is caused to flow downward through the annular channel 4 and is withdrawn through the exhaust tube 3. Any particles, which may result from the reaction, therefore deposit on the tube wall of the annular channel 4. Since the vertical wall of inner tube 1b is present between the annular channel 4 and the tubular space of inner tube 1b, into which and from which the wafer 8 enters and is taken out, the influence of the particles on the wafer 8 can be decreased.
Allegedly, a large diameter wafer, for example a 12-inch wafer, is necessary for producing a 64 M-DRAM in order to fulfill the multiple rule for cost reduction. Therefore, approximately 200 chips must be produced on a 12 inch wafer ("Nikkei Microdevice" November 1992). The integrating degree of a semiconductor device thus increases more and more. Since the particles have a coefficient of thermal expansion different from that of the quartz and, further, the adhesive force of the particles on the quartz is small, the number and size of the particles should be decreased as much as possible along with increase of the integration degree. This in turn makes it necessary to produce a semiconductor device in a small batch or in a treatment of a single wafer. The plant cost of an apparatus used for such production should not be increased.
Usually, when such treatment as CVD is carried out tens of times in a quartz reaction tube, the reaction product deposits thickly on the inner wall of the reaction tube to a thickness of approximately several tens of microns. The thus deposited reaction product peels due to the difference between the coefficient of thermal expansion of quartz of the reaction tube and that of the reaction product. In order to prevent the formation of particles, the reaction product deposited on the inner wall must be decomposed before it results in the formation of particles. The decomposition is carried out, for example, by plasma, which must be frequently carried out. This cleaning method is published in Japanese Unexamined Patent Publication No. 62-196,820, in which it describes a method to permanently install between a heater and the outer tube an electrode for generating plasma.
In the heat-treating method of a wafer disclosed in such a dual-tube type apparatus for producing a semiconductor device shown in FIG. 17, the reaction gas is caused to flow within the inner tube 1b, in which a wafer 8 is situated. The reaction therefore occurs within the inner tube 1b. As a result, the reaction product deposits somewhat on the inner wall of the inner tube 1b. The reaction occurs most actively at a location of the inner tube where the wafer 8 is positioned during the reaction, while the reaction occurs less actively deeper within the inner tube 1b where the wafer 8 is introduced and withdrawn. The superfluous reaction product deposits on the inner wall of the inner tube 1b. When the wafer 8 is withdrawn from the dual type reaction tube 1 by means of driving the mechanisms 30-33 described below, the wafer 8 is contaminated by the particles deposited on the inner wall of the inner tube 1b. The wafer 8 is also contaminated by the particles, when it is inserted into the dual type reaction tube 1. In FIG. 17, a magnet coil is denoted by 30, an outer tube is denoted by 31, and, a driving mechanism is denoted by 32. The wafer holder 6 is elevated or lowered by the mechanism 33, together with the shielding plate 11 and the pedestal 12.
According to a known method for cleaning the dual-tube type apparatus by plasma, the electrodes for cleaning are permanently installed between the outer tube and the heating furnace. The plasma generates, therefore, most intensively in the space between the inner and outer tubes, with the result that the inner wall of the inner tube is not cleaned effectively. Note that the inner wall should be cleaned, since the amount of deposition of the particle source there is great. The known cleaning method is therefore not satisfactory.